19.1 Standard hydrogen electrode
1 Describe the standard hydrogen electrode.
A standard hydrogen electrode is essentially a half-cell used as the standard in simple voltaic cells to measure the potential across all other half-cells.
2 Define the term standard electrode potential (E
The standard electrode potential (E
o) can be defined as the electric potential across a standard hydrogen electrode and a half-cell under standard conditions: 298k, 1 atm, 1 mol dm-3.
3 Calculate cell potentials using standard electrode potentials.
In order to work out the potential across two half-cells in a voltaic cell, you need to know the reaction occurring at each electrode.
You also need your IB chemistry data booklet. On page 12, it lists all the standard electrode potential for each redox reaction you know. It lists the reduction potential for each reaction i.e. the potential of the oxidized species being reduced by hydrogen.
To work out the cell potential (Ecell) of a cell, you just simply work out the difference in the standard electrode potential of the half-cells.
A voltaic cell features a copper half-cell and a zinc half-cell (Cu(s)/Cu2+ ll Zn(s)/Zn2+).
Standard electrode potential of each half-cell:
Zn2+ + 2e– → Zn = -0.76 V
Cu2+ + 2e– → Cu = 0.34 V
The difference between -0.76 and 0.34 is 1.10, therefore Ecell = 1.10 V.
4 Predict whether a reaction will be spontaneous using standard electrode potential values.
A reaction will be spontaneous when it has a positive value for potential i.e. release energy and becomes more stable in the process. The greater the value, the more reactive it is.
Looking at the standard electrode potential for zinc and copper:
Zn2+ + 2e– →Zn = -0.76 V
Cu2+ + 2e– →Cu = 0.34 V
The SHE value for zinc is negative; it is impossible for zinc to be reduced spontaneously by hydrogen.
However, the SHE value for copper is positive; copper will be reduced spontaneously by hydrogen.
Don’t forget that the values in the data booklet list the reduction potential for each reaction. If you want to determine whether not an oxidation reaction is spontaneous, you need to add a negative sign in front of the value.